The invention relates to a method and an arrangement for calibrating a rotary apparatus mounted on a movable part of a coordinate measuring machine.
It is known to use coordinate measuring machines (abbreviated to CMM) to determine coordinates of workpieces. Here, in some cases, a rotary apparatus is used which can bring the sensor, which is mounted on the movable part of the CMM, into different rotary positions. The sensor, which is in turn mounted on the rotary apparatus, may for example be a tactile probe for probing the surface of workpieces, that is to say, upon contact with the surface, using the measuring system of the coordinate measuring machine, the position of the tactile probe is determined (for example through the use of a measuring head which is mounted on the rotary apparatus and which bears the probe), and from this, the coordinates of the probed surface point are determined. Alternatively or in addition, however, other sensors, for example optical, capacitive and/or magnetic sensors, are arranged on the rotary apparatus in order to measure the workpiece.
A calibration of the rotary apparatus is necessary for various reasons. In particular, by means of the calibration, the geometry of the rotary apparatus and the relative position thereof and/or the relative orientation thereof with respect to the movable part of the coordinate measuring machine are determined or checked. These variables may however change over time and/or as a result of renewed mounting of the rotary apparatus on the coordinate measuring machine in relation to an earlier operating state. Changes in geometry (that is to say changes in shape and/or dimensions) over the course of time can be attributed to changes in temperature and to wear. The calibration should therefore be performed repeatedly.
The changes may have different effects in the different rotary positions. This may be due in particular to the fact that the orientation and/or position of the axis of rotation changes differently over the course of time for the different rotary positions, or the orientations and/or positions of multiple axes of rotation of the rotary apparatus change differently over the course of time for the various combinations of the rotary positions with respect to the multiple axes of rotation. In general, the rotary movements about the respective axis of rotation are not ideal rotary movements. For example, the orientation of an axis of rotation of the rotary apparatus changes during the rotational movement of those two parts of the rotary apparatus which are movable relative to one another about the axis of rotation.
Therefore, for the calibration of rotary apparatuses, it is customary for such rotary apparatuses to be brought into multiple different rotary positions and for measurement information for the calibration to be captured in each case. This increased measurement outlay is a major reason why the calibration is a lengthy process, which takes considerably longer still in the case of rotary apparatuses with multiple axes of rotation (for example so-called rotary/pivot joints) because, for the capture of the measurement information, different rotary positions with respect to one of the axes of rotation are combined with different rotary positions with respect to another axis of rotation. The long duration gives rise to additional inaccuracies of the calibration if the boundary conditions (in particular the temperature distribution) change during the calibration. Rotary/pivot joints are known for example from EP 1 624 282 A2.
It is known for a calibration body, in particular a sphere, to be fastened to a holding device which is positioned, along with the workpiece to be measured by means of the CMM, on a base of the CMM. In particular if the holding device and the calibration body are not influenced, or are influenced only to a minor extent, by temperature changes, the calibration body forms a good position reference. By means of a sensor mounted on the rotary apparatus, the calibration body is probed in tactile fashion at various points on its surface by the probe for calibration, or is scanned contactlessly, and the coordinates of at least one characteristic point of the calibration body are determined from the measurement values of the measuring system. Changes in the geometry of the rotary apparatus and changes in the relative position thereof and/or relative orientation thereof with respect to the movable part of the CMM, lead to apparent deviations of the position of the characteristic point from the expected position thereof. Consequently, the rotary apparatus can be calibrated utilizing the information regarding the deviations. A holding device for holding a calibration body, and a method for calibrating a measuring sensor of a CMM, are known for example from WO 2009/152962 A2.
Owing to the capture of measurement information at multiple surface points of the calibration body, the calibration outlay in the case of rotary apparatuses is particularly high. To take into account the changes in geometry, use may be made in particular of computing models. Such computing models commonly have a multiplicity of parameters, for the determination of which a large number of measurement values is required. For example, a computing model of said type is initialized by means of the calibration. In particular, starting values of the computing model are determined by means of the calibration. For example, by measurement of temperature changes over the course of time and/or measurement of temperature gradients or temperature differences at various locations, further information can be obtained for the computing model, and the change in geometry that has occurred since the initialization can be calculated by means of the computing model.
In particular, for the calibration of the rotary apparatus, use is always made of the same probe, which is connected to the rotary apparatus and by means of which the surface of the calibration body is probed.
It is an object of the present invention to specify a method and an arrangement for calibrating a rotary apparatus mounted on a movable part of a coordinate measuring machine, by means of which the outlay for the calibration can be reduced.